1. Field of the Invention
The invention is related to a receiver for optical digital signals wherein a transimpedance amplifier is provided connected to an output.
2. Description of the Prior Art
Such receivers for optical digital signals usually contain a transimpedance amplifier coupled to a photodiode, and also contain a following comparator which emits voltage pulses in accordance with the received light pulses. For example, the comparator output assumes a logical "1" when the photodiode is illuminated, or vice versa.
European Patent Application No. 0 052 221 discloses a transimpedance amplifier which is constructed as an operational amplifier. For conversion into binary voltage signals, this operational amplifier is followed by a comparator.
The article "Transimpedance Optical Preamplifier Having A Common Collector Front End", printed in Electronics Letters, Vol. 18, No. 23, Nov. 11 1982, discloses a discretely constructed transimpedance amplifier which has three amplifier stages each of which comprises a transistor.
In such dc-coupled receivers for optical digital signals, the switching of the output signal emitted by the comparator is dependent on the absolute value of the current which flows through the photodiode. The comparator can not distinguish whether the current flowing through the photodiode is a photocurrent due to an illumination of the photodiode, or is a dark current of the photodiode which increases due to an increase in temperature.
The dark current of the photodiode, which is within an order of magnitude of 1 to 10 nA at room temperature, increases exponentially with the increase in temperature and reaches values of about 100 nA at 80.degree. C. This means that a receiver already has its function disturbed by such a high photodiode dark current.
It is possible to limit the ambient temperature of the receiver to values at which the dark current of the photodiode is still adequately low.
It is further possible to operate the photodiode with a bias voltage of 0, i.e. no dark current can flow. However, the transmission rate for the optical digital signals is thus limited, and the sensitivity of the photodiode is reduced.
It is also possible to reduce the sensitivity of the receiver to such a degree that the dark current of the photodiode is far lower than the photocurrent required for switching even at the highest occurring ambient temperature.
The "Telecommunications Databook" of National Semiconductor Corporation 1984, pp. 5-23, discloses an optical receiver which comprises two amplifiers each coupled to a photodiode. Only one of the photodiodes is illuminated. The outputs of the amplifiers are connected to a comparator.